Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.
The amount of light generated by an LED is generally proportional to the current applied to the LED. As more current is applied, more light is generated. However, as the current density applied to an LED increases, the external quantum efficiency of the LED initially increases to a peak at fairly low current density, then decreases at high current density. Accordingly, many LEDs operate most efficiently at fairly low current density.
Some applications such as, for example, automotive applications, require high light output in a relatively small source size. In order to generate the light output required, a single LED must be operated at a current density that is generally higher than the current density at which the LED is most efficient, in order to generate enough light. Because the source size must be small, in some applications it is not possible to generate the needed light output with multiple LEDs operated at lower current density, placed side by side.